1. Field of the Invention
The invention relates to a sweep-type fingerprint sensor module and a sensing method therefor, and more particular to a sweep-type fingerprint sensor module having a speed-detecting unit and a sensing method therefor.
2. Description of the Related Art
There are many known techniques of identifying an individual through the identification of the individual's fingerprint. The use of an ink pad and the direct transfer of ink by the thumb or finger from the ink pad to a recording card is the standard way of making this identification. Then, an optical scanner scans the recording card to get an image, which is then compared to fingerprint images in the computer database. However, the most serious drawback of the above-mentioned method is that the fingerprint identification cannot be processed in real-time, and thus cannot satisfy the requirement of real-time authentication, such as network authentication, e-business, portable electronics products, personal ID card, security system, and the like.
The method for reading a fingerprint in real-time has become the important technology in the biometrics market. Conventionally, an optical fingerprint sensor may be used to read a fingerprint in real-time. However, the optical fingerprint sensor has a drawback because it is large in size. Consequently, silicon fingerprint sensors, which overcome the drawbacks of the optical sensor and are formed by silicon semiconductor technology, are developed.
Owing to the finger dimension, the sensing area of the conventional silicon fingerprint sensor is large, for example, it is greater than 9 mm*9 mm. Furthermore, owing to the limitations in manufacturing the silicon integrated circuit, only 50 to 70 good dies may be formed in a 6″ wafer. The price of a single fingerprint sensor is greater than at least 10 U.S. dollars when the packaging and testing costs are included. Thus, this expensive price may restrict the silicon fingerprint sensor in various consumer electronics applications such as notebook computers, mobile phones, personal digital assistants, computer peripheral product, or even personal ID cards embedded with the fingerprint sensor.
Consequently, it is possible to reduce one-dimensional width of the conventional, two-dimensional (2D) silicon fingerprint sensor so as to increase the number of good dies and decrease the price of the sensor chip. In this case, the finger sweeps across the sensor surface and the overall finger is sequentially scanned into plural fingerprint fragment images, which are then stitched into a complete fingerprint image.
Mainguet discloses a sweep-type fingerprint sensor formed by pyroelectric or piezoelectric material (pressure-type) and a method for re-arranging plural fragment images in U.S. Pat. No. 6,289,114. However, the method has to disadvantageously acquire many fingerprint fragment images and to re-construct them into a complete fingerprint image because the sensor cannot determine the sweeping speed of the finger. In this case, the microprocessor has to be powerful, the capacity of the memory has to be great enough, and very probable errors may happen due to reconstruction of so many fingerprint fragments.
In the conventional sweep-type fingerprint sensor, the number of sensing members in the Y-axis direction (the sweeping direction of the finger) is preferably decreased in order to reduce the sensor area. In this case, the dimension of the sensor in the Y-axis direction ranges from 0.8 to 1.6 mm. Thus, the number of acquired fingerprint fragment image is great.
FIG. 1 is a schematic illustration showing a finger sweeping across a conventional sweep-type fingerprint sensor. As shown in FIG. 1, a finger 120 sweeps across a sweep-type fingerprint sensor 110 in the Y-axis direction with a speed V, and the sweep-type fingerprint sensor 110 acquires a plurality of fingerprint fragment images.
FIG. 2 is a schematic illustration showing plural fingerprint fragment images acquired by the fingerprint sensor of FIG. 1. As shown in FIG. 2, the acquired fingerprint fragment images II(1) to II(N) may be stitched into a complete fingerprint image of the finger 120. Because the sensing dimension along the sweeping direction of the conventional sweep-type fingerprint sensor is too small, tens to hundreds of images II(1) to II(N) have to be stored. The prior art only acknowledges to design the sweep-type fingerprint sensor with reduced area of the sensor (reduced number of sensing members in the Y-axis direction), so the Y-axis dimension of the sensing members array mostly ranges from 0.8 to 1.6 mm. Furthermore, since the conventional fingerprint sensor does not detect the sweeping speed of the finger and has to take a safety factor into consideration, the overlapped regions AA(1) to AA(N−1) between adjacent images are too large, thereby increasing the number of fragment images and complicating the stitching process.
Regarding about the above-mentioned drawbacks, it is quite difficult for the conventional sweep-type fingerprint sensor module to have minutia points extraction from a recombined fingerprint image due to such many fragment images without errors happening. Therefore, a method in realistic application is to transfer the acquired gray-scale images into the frequency domain, and the advantage of this method resides in that it does not need a lot of image processing steps. However, this method tends to cause high error rate due to the humidity of the finger or contamination on the chip surface.